docs: def subsample(self, sample_factor=2):
          """
          Subsample series by an integer factor.
  
          Parameters
          ----------
          sample_factor : positive integer, optional, default=2
              Factor for downsampling.
          """
          if sample_factor < 0:
              raise Exception('Factor for subsampling must be postive, got %g' % sample_factor)
          s = slice(0, len(self.index), sample_factor)
          newindex = self.index[s]
          return self.map(lambda v: v[s], index=newindex)